Method of separating ions of different specific charges



w. PAUL EIAL 2,950,389

METHOD OF SEPARATING IUNS OF DIFFERENT SPECIFIC CHARGES Aug. 23,1960

3 Sheets-Sheet 1 Filed Dec. 24, 1958 5 4 4. 3 5 0 0 3 0 0 5 K 1 2 0 8 2I 5 W 0 Q o o 00 l 5 l5 4 0 l4 0 6 5 0 3 O M M E O 4 0 w I O 0 O 5 O ll2 1 a 0I O k u Fig.1

Aug. 23, 1960 w. PAUL El'AL 2,950,389

METHOD OF SEPARATING IONS OF DIFFERENT SPECIFIC CHARGES Filed Dec. 24,1958 3 Sheets-Sheet 2 2b 1 2a: 2a

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Aug. 23, 1960 w. PAUL ETAL 2,950,389

METHOD OF SEPARATING IONS OF DIFFERENT SPECIFIC CHARGES 3 Sheets-Sheet 3Filed Dec. 24, 1958 METHOD OF SEPARATING IONS OF DIFFERENT SPECIFICCHARGES Wolfgang Paul and Hans-Peter Reinhard, Bonn, and

Heinz Frohlich, Erlangen, Germany, assignors of onehalf toSiemens-Schuckertwerke Aktiengesellschaft, Erlangen, Germany, acorporation of Germany, and one-half to said Wolfgang Paul Filed Dec.24, 1958, Ser. No. 782,838

'12 Claims. (Cl. 250-413) Our invention relates to a mass-spectrometermethod for the separation or separate indication of ions of respectivelydifferent specific electric charges and, more particularly, to a methodbased upon the principle known from the German Patent No. 944,900 anddisclosed in US. application Serial No. 476,812 filed December 21, 1954.According to that principle the ions are shot into a periodicallyvarying electric field whose potential go is a square function of thecoordinates x, y, z of the general where e is the electric charge and mthe mass of the particles. The ions that follow a stable trajectory passthrough the electric field onto a collector electrode or other target,whereas the ions following instable trajectories impinge upon thelaterally located electrodes that produce the electric field, thus beingprevented from reaching the target. In this manner the desired isotopeseparation or separate indication is obtained.

The above-mentioned periodic function 1"(t) may be constituted, forexample, by a sinusoidal oscillation superimposed upon a constant finitevalue. Suitably shaped electrodes serve for producing the electric fieldof the above-mentioned periodic potential. In the special case of thesinusoidal field potential just mentioned, the electrodes are impressedby a constant direct voltage and also by a sinusoidal voltage of highfrequency. As a result, there result stable ranges in which theoscillation amplitude of ions of a given specific electric charge doesnot exceed a given maximum value. Hence, only such ions can pass fromthe ion source between the electrodes to the target. The other ions,having different specific electric charges and performing instableoscillations after entering the periodic electric field, assumeoscillation amplitudes of such large magnitude as to impinge upon theelectrodes. For further explanation of these phenomena, reference may behad to the above-mentioned German patent. As also disclosed in thepatent, a narrow instable range can be embedded in a wide stable rangeby superimposing upon a rotationally symmetrical highfrequency fieldanother alternating field of smaller amplitude or potential whosefrequency is one-half of that of the high-frequency field. This issupposed to afford nitecl States atent O ice a separation of ions of agiven charge out of an isotope mixture.

However, when using a superimposed alternating field of one-half of thefrequency of the high-frequency main field, the desired results can beattained only for a very limited number of practical applications.

It is therefore an object of our invention to devise a method of isotopeseparation which, based upon the prin ciple above mentioned, is moregenerally applicable and can be used more reliably with rotationallysymmetrical as well as non-rotationally symmetrical electric fields.

Another object of the invention is to provide a method and means capableof simultaneously separating two or more isotopes, i.e. ions ofrespectively different specific charges, from an isotope mixturecomprising more than the two isotopes to be separated.

Still another object is to devise a method and means for converting anisotope mixture of a given composition to a desired differentcomposition with respect to the relative'proportions of the differentlycharged ions.

In accordance with our invention, we pass a flow of differently chargedions through a periodic high-frequency field substantially as known, butsuperimpose upon that field another alternating field whose frequency isadjusted to the selected working point within the stable range. Morespecifically, the superimposed additional alternating field is given afrequency which coincides at least approximately with the fundamental orupper harmonic oscillation of ions having a predetermined specificcharge, thus enforcing instable paths for these particular ions.

The fundamental oscillation frequency of ions of a given specificelectric charge, traveling through an electric field comprised of aconstant unidirectional component and a periodically variablehigh-frequency component, depends not only upon said electric charge ofthe ions but also upon the magnitude of the field-producing directvoltage, the amplitude of the field-producing highfrequency voltage, themagnitude of the high-frequency itself, and also upon the geometry ofthe field-producing electrodes. Since these data are known for any givendevice, the fundamental oscillation frequency of the ions of a givencharge can readily be determined for any given working point. Thisfundamental frequency is proportional to the high frequency, theproportionality factor k being determined by the chosen working pointWithin the stable range. The upper harmonics in each case are the directresult of the sum or difference of integral multiples of the high fieldfrequency on the one hand, and the fundamental oscillation frequency ofthe ions on the other hand.

The invention will be further described with reference to the drawingsin which:

Fig. l is an explanatory graph relating to the method as performed bymeans of apparatus as shown in Figs. 2 and 3;

Fig. 2 is a longitudinal and sectional view of an apparatus for isotopeseparation according to the invention;

Fig. 3 is a cross section of the apparatus along the line III-III inFig. 2;

Figs. 4 and 5 are explanatory and show two different ways of applyingthe necessary field voltages to the electrodes of the apparatus; and

Fig. 6 is an electric circuit diagram of the components for producingthe field voltages.

Fig. 7 illustrates one form of elongated electrodes of the hyperbolictype.

The graph shown in Fig. 1 is a so-called stability diagram for acylinder-symmetrical electrode arrangement whose symmetry axisconstitutes the z axis. The magnitudes a and q represented by thecoordinate axes of J? the diagram determine the working point and resultfrom the data of the device as follows:

The stability rangeis indicated in. the diagram by a heavy line/ Shownwithin the range so indicated are marker lines for theproportionalityfactors k. The full marker lines apply to the xcomponent, and the broken marker lines to the y component of the iontrajectory. The fundamental and upper harmonics are different for thetwo components, whichmustbetaken into consideration when performing themethod according to the invention. V V

The above-described method of the invention can be modified forseparating ions of several given, different electric charges from a morecomprehensive mixture of isotopes. For thispurpose, a plurality ofadditional alternating fields of correspondingly different frequenciesare simultaneously superimposed upon the combined constantunidirectional and high-frequency fields.

It has been found that the tuning of the frequency of the superimposedfield to the fundamental or upper harmonic frequencies of the ions neednot be strict. In

A the eventfoffrequency differences, the traveling ions oscillate alongtheir trajectory at beat frequencies with amplitudes that may become solarge as to result in the desired separation.

.According to another feature of our invention, advantage is taken ofthis phenomenon for simultaneously separating ions of respectivelydifferent specific charges, without the need for using as manysuperimposed frequencies as there are different electric charges.Accordingly, the separation is effected by superimposing one or morealternating fields whose frequencies, at least with respect to some ofthese fields, are between the fundamental or upper-harmonic oscillationfrequencies of the difierently charged ions to be separated.

The ranges of the respectively different electric charges of theisotopes to be separated need not be contiguous. For example, ions of agiven specific charge can be sep arated by superimposing an alternatingfield Whose frequency corresponds to an upper harmonic of these ions,whereas ions of a different charge are simultaneously separated by meansof a superimposed frequency corresponding to the fundamental frequencyof the latter ions, as long as these different oscillation frequenciesare relatively close to each other. In such cases, it is' also possibleto utilize the fact that the oscillation frequencies for the individualcomponents (y, x) are different. The fundamental and upper harmonicfrequencies of the ions not to be separated must be sufliciently remotefrom the frequency of the superimposed alternating field.

By selecting the frequencies and/or the amplitudes of the superimposedalternating fields, the beat-frequency amplitudes for the ions of therespectively different electric charges can be separately determined.The higher the amplitude of the superimposed fields is chosen, thebroader will be the range in which the separation takes place in eachcase. This offers the possibility to convert a given'isotope mixture bymeans of a single operating process to a desired different composition.In this case, individual quantities of ions can be fully separated fromthe mixture, or all components of the mixture are varied only with,respect to their relative proportions. This possibility of simply andrapidly changing the composition.

passing through the container wall.

4 of a mixture ofisotopes represents a particular advantage of themethod according to the invention.

The above-mentioned features of the invention will be more fullyunderstood from the following description of the apparatus shown inFigs. 2 to 6 and from the numerical examples given further below.

The apparatus illustrated in Figs. 2 and 3 comprises an evacuable.vessel 4 in which four cylindrical. rod: shaped electrodes 1 aresymmetrically mounted in parallel relation to each other. Each twoelectrodes, located diametrically opposite each other, are mutuallyspaced a distance, equal to the electrode diameter. Instead of givingeach electrode a strictly circular cross section, it

. may also be given a hyperbolic surface at least at the side facing theother electrodes, as shown in said German Patent No. 944,900, issuedJune 28, 1956. The electrodes are kept inproper positionrelativeto eachother and to the vessel by means of insulating discs 4 of ceramicmaterial and are preferably adjustable.

Thedirect and "alternating voltages required for producing the electricfield between the electrodes are supplied thereto by conductorsZ whichare located in housings 2a vacuum-tightly connected with the vessel 4 bymeans of screw caps 211. A conventional ion source S, comprisingashockion generator or alow-voltage arc discharge, is. joined and hermeticallysealed with the vessel 4 by meansof flanges 4a, 5a. From source 5, theions are shot in the direction of the arrow 6 into, and

axially along, the fieldspace between the electrodes with 30' a givenkinetic energy. "The ions having a stable trajectory pass through theentire axial length of the electric field and reach the collectorelectrode 3 which is grounded through an external resistor 7 by a leadvacuum-tightly The voltage drop of resistor 7 due to the discharge fromelectrode 7 to ground can be measured, for example by means of avoltmetric device 7a. Located at both axial ends of vessel 4 arecircular diaphragms 8 which have respective center openings for thepassage of the ions and which shield the ionsource-and-the collectorelectrode 3 from the highfrequency field between the electrodes 1. Asmentioned above, those ions that are excited by the high frequency fieldto oscillate along their ftrajectory with unlimited impinge upon theelectrodes 1.

The. direct voltages and alternating voltages for producing the fieldare supplied to the electrodes from the generating j componentsillustrated in Fig. 6 in accordmice with one of the diagrams shown inFigs. 4 and 5. For better distinction, the electrodes denoted by 1 inFigs. 2 and 3: are designated in Figs. 4 and 5 by A A B and B Impressedupon these four electrodesare the voltages mentioned in the foregoinggeneral description of the'method according to the invention and morefully specified hereinafter. Supplied to each electrode is a directvoltage as well as a superimposed alternating voltage or high frequencyfor producing the main guiding field. As schematically shown in Fig. 4,the electrodes A and A are'further supplied with one or two additionalsuperimposed alternatingvoltages. However, the supply of the alternatingvoltages may also be such that a first additional alternating voltage isapplied to the electrodes A and A and a second alternating voltage tothe'electrodes B and B as is shown in Fig. 5. The terminals a to aaccording to Fig. 4 or Fig. 5 are conductively connected withthecorrespondingly designated terminals respectivelyof the circuitdiagram according to Fig. 6.7 r

According to Fig. 6, a high-frequency generator G -for..the. mainguiding field acts through a transformer coupling and apush-pull finalstage GE upon an inductance coil S The coil S forms together witheapacitors C C C and the capacitance of the electrodes relative .to eachother (B and-B relative to A and A a resonance circuit tuned to thefrequency of the generator amplitude, cannot reach the collectorelectrode 3 but G Two tank circuits formed of capacitors C C andinductance coils D D D on the one hand, and capacitors C C and coils D DD on the other hand, both being likewise tuned to the frequency of thehighfrequency generator G form a short circuit for the electrode pairs AA and B B with respect to the high frequency of the main guiding field.Inductively coupled with the inductance coil S is another inductancecoil S whose alternating voltage is rectified by a diode Di. Thefiltered direct voltage is adjustable by means of potentiometerresistors P and P and thus can be placed into different fixed relationsto the amplitude of the highfrequency voltage of the main guiding field.The direct voltage is connected to terminals a to a; through blockingreactors D D and D D With an electrode connection according to Fig. 4,the electrodes A and A are connected to one pole of the direct voltage,and the electrodes B and B are connected to the other pole.

A second frequency generator G for a first resonance field acts upon aresonance transformer T which forms a tank circuit together withcapacitors C Cg and together with the mutual capacitance of two oppositeelectrodes. This tank circuit is tuned to the frequency of the generatorG Connected in parallel to capacitors C and C are terminal and dampingresistors R and R The resonant connecting circuit formed by capacitors Cand C with inductance coils D and D is likewise tuned to the frequencyof the generator G and acts together with a grounded reactor D to blockthe voltage of the main guiding field relative to ground.

In the two connecting possibilities according to Figs. 4 and 5, theelectrodes A and A are connected to the terminals a a and 11 arespectively.

The above-mentioned reactors D to D and the capacitors C and C in thecircuit of the high-frequency generator G serve to prevent the voltageof the superimposed first resonance field from being short circuited.

Another frequency generator G is provided for producing a secondresonance field. The generator G supplies its voltage to the terminals11-; and a by a circuit connection corresponding in design and operationto that described above with reference to generator G The operation ofthe network connection for generator G is also identical with thatdescribed above for generator G the resonance circuits C D and C D beingtuned to the frequency of generator G The generator G need not in allcases be connected to the electrodes. On the other hand, additionalfrequency generators (not illustrated) may be connected with theelectrodes by circuit connections analogous to those described above.

Fig. 7 illustrates the hyperbolic type of elongated electrodes mentionedabove, and described in said German patent. The opposite pairs aredesignated A, A and B, B respectively.

In summary, it will be recognized that the electrodes are impressed withcomponent direct voltage (from Di) and component high-frequency voltage(from G for producing the main guiding field, and that one or moreresonance fields of predetermined, respectively different frequenciesare superimposed by means of one or more additional generators (G G Withreference to Figs. 4 and 5, the illustrated terminals a to a may eitherall be connected to the frequency generators according to Fig. 6 or,instead, the terminals a and a and the terminals a and a may not beconductively connected with the corresponding frequency generators. Incase all terminals a to a are to be active, the connection according toFig. 5 is preferable, as a rule, if the frequencies of both resonancefields are very close to each other.

It will be recognized that the above-described apparatus according toFigs. 2 to 6 is also suitable for permitting a selection of thefrequencies or a selection of the amplitudes of the additionallysuperimposed alternating fields, without necessitating any change in thei1- lustrated devices. The adaptation to the particular se lectedfrequencies is effected merely by correspondingly changing the tuning ofthe resonant connecting circuits, this being apparent from the provisionof variable capacitances such as those denoted by C C C The methodperformed by the operation of the apparatus will be further explainedpresently.

in a system of rectangular coordinates x, y, z, the motion of asingle-charge corpuscle (ion) which is subjected to a variable voltage(1) 0=U+Vcos wt can be represented by the following equations for thethree individual components of motion:

The terms used in these equations have the following meaning:

mutually opposite, cylindrical electrodes each having the radius 1*.

frequency of the alternating voltage If one sets: (5) 1 T wi=2s theequations (2) and (3) convert to the normal form of Mathieusdifferential equations:

(10) x"+ [a-2q cos (ZS-I-rr) 1x=0 (11) y ia+2q cos 2s] -y=0 Consequentlythe solutions, for example of Equation 10, have within the stable rangethe form The starting conditions (radiation diameter and aperture angleof the enclosed ion beam) and the maximum oscillation amplitudesdetermined by these conditions, are given by the magnitudes of theconstants A and B. The starting conditions must be so chosen that thefinite maximum amplitude of oscillation remains smaller than theelectrode spacing 2r because otherwise ions with stable trajectorieswill also be separated.

The coefficients c and 5 depend only upon the field magnitudes and henceupon a and q. Ions of the same mass number differ, at a given .pointunder observation,

determined by interpolation, and

2:4.8-10 (electrostatic charge units) I, 10

a= 0.112 q=0.66 r=1.5 Cm.

' w -1.5 megaeyclesper-sec- When using the above-mentioned specificvalues in a deviceas described above with reference to Figs. 2 to 6,

the values a and q are located in the middle of the stability diagramofvFig. 1; and the value.

represents astraight line which starts from the zero point andintersects the diagram in two points. These two pointsrhave thecoordinates a =0.055, -0.335, and

The masses m and m can be calculated from Equations 5 and 6 by using theabove-given values for U, r, w, and 2. It results that m =l.67-10- -MAll masses between these. two are stable and travel from the ion sourcethrough the interspace between the electrodes onto the collectorelectrode. Now, if the mass of mass number M 200, located within thementioned range of masses, is to be separated, then one eleca trode pairmust be caused to produce a resonance alternating field with "aresonance frequency corresponding to the oscillation frequency of thisparticular mass. As a result, the ions of the mass M=2OO are graduallyexcited by resonance to oscillate with such a large amplitude that theyreach the field electrodes and thus are eliminated from the ion beam.The resonance frequency can be determined from the solutions of theabovementioned Mathieus differential equations. The natural frequency ofthe ion in the guiding field of the frequency w amounts to w ==(/L:|:g w

and can be determined for the fundamental oscillation (11:0) 3.8

It follows that the frequency of the fundamental oscillation of the ionalong its trajectory is:

The value tom is entered in Fig. 1 for the x and y directions with thedesignation k and k the z-direction extending along the longitudinalsymmetry axis of the electrode assembly. For occurrence of resonance inthe x-direction of an ion having the mass number M= 200, theintersection between a and g applying to this particular mass can be onethus obtains the value k ;0.327.

The necessary resonance frequency is The amplitude V of the resonancefrequency may be chosen to be-approximately 2% of the voltage V. Thiscorresponds, in the exampleherebeing discussed;- to- V =280 volts.Thepercentage just given has been-determined by experiments.

gether with the one ofprimary interest.

To performthe separation with the aid "ofthe desired economical amount0f power N, the best suitable values can-be determined by tests, Thepower- N canbe determined from -tl1e;semi-empiric equation wherein Cdenotes the capacitance of the electrodes in [F] (Farad), and 0denotestheresonance quality.

Wi-th a length L=600 cm. of the electrodes andan electrode diameter ofr=1.5 cm., thevalue for C can be determined from theEquation' 13 asC=9'-10- [F]. By

selecting.0=200, one obtains N =4.2 kw.

The ions issuing from'the'ion source are subjected to a givenaccelerating voltage U (between ion sourceand grounded collector) whichimparts to the ions the necessary and correct velocity at their entranceinto the elec-.

troderange.

The semi-empirical equation for the accelerating voltage U3 is t 14 W 6with the beat frequency volts consideration that the necessary platevoltage of the device is U =900 volts.

The permissible current flow from the ion source t through theelectrodegap follows from Gauss theorem,

concerning the field strength produced by the space charge, The formulafor the permissible current is IU 5. I 4.9 10 r V M milhamps wherein ris given in cm., V in volt, and U in volt. Equation 15 is based upon aconstant space-charge density, resulting in a maximal space-charge fieldstrength of The separation ofadjacent masses (adjacent isotopes)effected in this manner is approximately 50%.

The foregoing explanations relating to the, fundamental oscillations ofthe ions along their trajectory apply also for upper harmonics, it beingonly necessary to determine 7 the particular resonance frequency fromferent specific electric charges, which comprises passing the ionsthrough an electric field having a field potential periodically varyingin accordance with a square function of the space coordinates (x, y, z),said function having the form Whenlarger amplitudes are a chosen,toomany neighboring massesare separated to- 9 wherein Kt) is a periodicfunction of time and B, 'y are positive constants satisfying theequation u+fi=' and superimposing upon said field another alternatingfield whose frequency coincides approximately with an oscillationfrequency of ions of pre-selected specific charge, whereby said ions ofselected charge are forced to travel on instable trajectory paths.

2. The method of separating ions of respectively different specificelectric charge, which comprises passing the ions through an electricfield formed between mutually spaced electrodes and having a constantcomponent and a high-frequency component, said field having a potential((p) of the general form wherein f(t) is a periodic function of time,and x, y, z are space coordinates, and 0:, B, 'y are positive constantssatisfying the equation a+f3='y; and superimposing upon said field analternating field of lower frequency than said high-frequency component,said superimposed frequency coinciding approximately with an oscillationfrequency of ions of pre-selected specific charge, whereby said ions ofselected charge are forced to travel on instable trajectory paths.

3. In the method according to claim 2, said frequency of saidsuperimposed alternating field being approximately coincident with thefundamental oscillation frequency of said ions of pre-selected specificcharge.

4. In the. method according to claim 2, said frequency of saidsuperimposed alternating field being approximately coincident with anupper harmonic oscillation frequency of said ions of pre-selectedspecific charge.

5. The method of separating ions of respectively different specificelectric charges from an isotope mixture, which comprises passing theions through an electric highfrequency field having a potential of theform wherein (t) is a periodic function of time, and x, y, z are spacecoordinates, and a, {3, 'y are positive constants meeting the equationu|/3='y; and superimposing upon said high-frequency field a plurality ofalternating fields of respectively different frequencies correspondingapproximately to the oscillation frequencies of ions of predetermineddifierent specific charges respectively, whereby said latter ions aresimultaneously separated from the isotope mixture.

6. In the method according to claim 5, said superim posed alternatingfields comprising at least one field having a frequency between thefundamental oscillation frequency of the ions of one given specificcharge and an upper harmonic frequency of the ions having another givenspecific charge.

7. In the method according to claim 5, said superimposed alternatingfields having predetermined difierent amplitudes respectively forobtaining respectively different degrees of separation for said ions ofrespectively different predetermined specific charges.

8. An apparatus for separating ions of respectively different specificelectric charges by causing the ions to assume oscillations havingamplitudes correlative with the specific charges, comprising a vacuumvessel, an ion source and a collector electrode axially spaced from eachother in said vessel, two pairs of elongated field electrodes extendingin said vessel between said source and said collector electrode, saidelectrodes presenting substantially cylindrically curved surfaces insymmetrical and radially spaced relation to the common axis thereof, thespacing apart of said surfaces being predetermined so that ions havingthe larger oscillation amplitudes impinge on said electrodes, a sourceof component direct voltage and a source of component high-frequencyvoltage jointly connected to said field electrodes to provide aresultant main guiding field between said pairs of electrodes, andcircuit means providing a source of alteri0 natin'g voltage of a lowerfrequency connected to at least one of said electrode pairs, and meansfor tuning the frequency of the latter voltage to resonance with theoscillations imparted by said main guiding field to an ion of a selectedspecific electric charge so as to augment the tendency of said ion toimpinge upon said field electrodes.

9. Apparatus for separating ions of respectively different specificelectric charges by causing the ions to as sume oscillations havingamplitudes correlative with the specific charges, comprising a vacuumvessel, an ion source and a collector electrode axially spaced from eachother in said vessel, two pairs of elongated field electrodes extendingin said vessel between said source and said collector electrode, saidelectrodes presenting substantially cylindrically curved surfaces inradially spaced relation to the common axis thereof, the spacing apartof said surfaces being predetermined so that ions having the largeroscillation amplitudes impinge on said electrodes, a source of componentdirect voltage and a source of component high-frequency voltage jointlyconnected to said field electrodes to provide a resultant main guidingfield between said pairs of electrodes, and a plurality of alternatingvoltage sources connected to at least one of said electrode pairs andhaving different frequencies, lower than that of said high-frequencycomponent, and means for tuning the latter frequencies to resonance withthe oscillations imparted by said main guiding field to ions of selectedand respectively different specific electric charges so as to augmentthe tendency of said ions to impinge upon said elongated electrodes.

10. An apparatus for separating ions of respectively different specificelectric charges by causing the ions to assume oscillations havingamplitudes correlative with the specific charges, comprising a vacuumvessel, an ion source and a collector electrode axially spaced from eachother in said vessel, two pairs of elongated field electrodes extendingin said vessel between said source and said collector electrode insymmetripal and radially spaced relation to the common axis thereof,said electrodes being substantially hyperboloid in transverse sec tion,the spacing apart of the hyperboloid surfaces being predetermined sothat ions having the larger oscillation amplitudes impinge on saidelectrodes, a source of component direct voltage and a source ofcomponent high-frequency voltage jointly connected to said fieldelectrodes to provide a resultant main guiding field between said pairsof electrodes, and circuit means providing a source of alternatingvoltage of lower frequency connected to at least one of said electrodepairs and means for tuning the frequency of the latter voltage toresonance with the oscillations imparted by said main guiding field toan ion of a selected specific electric charge so as to augment thetendency of said ion to impinge upon said field electrodes.

11. Apparatus for separating ions of respectively different specificelectric charges by causing the ions to assume oscillations havingamplitudes correlative with the specific charges, comprising a vacuumvessel, an ion source and a collector electrode axially spaced from eachother in said vessel, two pairs of elongated field electrodes extendingin said vessel between said source and said collector electrode insymmetrical and radially spaced relation to the common axis thereof,said electrodes being substantially hyperboloid in transverse section,the spacing apart of the hyperboloidal surfaces being predetermined sothat ions having the larger oscillation amplitudes impinge on saidelectrodes, a source of component direct voltage and a source ofcomponent highfrequency voltage jointly connected to said fieldelectrodes to provide a resultant main guiding field between said pairsof electrodes, and a plurality of alternating Voltage sources connectedto at least one of said electrode pairs and having different frequencieswhich are lower than said high-frequency component, and means for tuningthe latter frequencies toresonance with the oscillations imparted bysaid main guiding field to ions of-selected and respectively differentspecific electric charges so as'to augment the tendency of said ions toelectrodes'extending in said vessel between said source and saidcollector electrodein symmetrical and radially spaced relation to thecommon axis thereof, the spacing apart of said surfaces beingpredetermined so that ions having the larger oscillation amplitudesimpinge on said'electrodes, a source of component direct voltage and asource of component high-frequency voltage jointly connected to saidfield electrodesto provide a resultant main guiding field between saidpairs of electrodes to produce an electric field having a fieldpotential ((p') periodically Varying in accordance with a squarefunction of the space coordinates -(x, y, z), vsaid function having:

wherein f(t) is a periodic function of time and a, B, are positiveconstants satisfying the equation l-5:

and circuit means providing a source of alternating voltage of a lowerfrequency connected to at leastone of said electrode pairs, and meansfor tuning the frequency of the latter voltage to resonance withthe'oscillations imparted by said main guiding field to an ion of aselected specific electric charge so as to augment the tendency of saidion to impingeupon said field electrodes.

References Cited in the file ofthis patent UNITED STATES PATENTS2,769,910 Elings Nov. 6, 1956 ,818,507 Britten Dec. 31, 1957 2,879,439

Townes Mar. 24, 1959 UNITED STATES PATENT OFFICE CERTIFICATION OFCORRECTION Patent No. 2,950,389 August 23 1960 Wolfgang Paul et alo thaterror appears in the above numbered pat- It is hereby certified (1 thatthe said Letters Patent should read as ent requiring correction ancorrected below.

In the heading to the printed specification, between lines 9 and 10,insert- Claims priority application Germany December 27 1957 Signed andsealed this 25th day of April 1961'.

(SEAL) Auest:

DAVID L, LADD ERNEST W SWIDER Commissioner of Patents Attesting Officer

